The mammalian vomeronasal organ (VNO) and accessory olfactory bulb is considered to be a relatively recent evolutionary recent evolutionary modification to the complement of chemosensory systems used by vertebrates for odorant/pheromone detection. However, a newly-identified family of fish odorant receptors, one of which detects the amino acid L- arginine, has been described as structurally homologous to the tetrapod V2R receptors expressed in the VNO. These findings suggest that elements of the tetrapod VNO are present in non-tetrapods. For over 80 years, two distinct olfactory pathways have been recognized in fish. The medial pathway of the fish olfactory system is proposed as analogous to the tetrapod accessory pathway on the basis of processing of pheromone input. However, in fish V2R receptor input is likely to project to the lateral pathway suggesting that this pathway may be structurally homologous to the tetrapod accessory olfactory pathway. We propose to functionally characterize processing of olfactory input by the medial and lateral pathways of the zebrafish olfactory bulb with the intent to identify similarities and differences in projection neuron (mitral cells, ruffed cells) and interneuron (granule cells, tyrosine hydroxylase-positive cells) activation and interactions between the medial and lateral pathways. Our experimental strategy relies on an immunoreactive ion channel permeant probe (AGB) which enables us to measure the activity of ionotropic glutamate receptors. Consequently, we must first establish that glutamate is the excitatory neurotransmitter used by these pathways, then establish the functional distribution of ionotropic glutamate receptors (Aim 1). The second aim will establish the patterns of odor-stimulated projection neuron and local interneuron activation in medial and lateral pathways, the extent of interaction between these pathways and the role of lateral inhibitory signaling. The third aim will establish the role of lateral inhibitory signaling. The third aim will establish the role of odor environment in the functional plasticity of the olfactory system. Completion of the proposed aims will provide the essential anatomical and physiological data necessary to establish the relationship between lateral and medial processing by non-tetrapods and main and accessory olfactory processing by tetrapods.